Contact or proximity printing using a magnified mask image

ABSTRACT

Improvements in the fabrication of integrated circuits are driven by the decrease of the size of the features printed on the wafers. Current lithography techniques limits have been extended through the use of phase-shifting masks, off-axis illumination, and proximity effect correction. More recently, liquid immersion lithography has been proposed as a way to extend even further the limits of optical lithography. This invention described a methodology based on contact or proximity printing using a projection lens to define the image of the mask onto the wafer. As the imaging is performed in a solid material, larger refractive indices can be obtained and the resolution of the imaging system can be increased.

FIELD OF THE INVENTION

The invention relates to the process of fabricating semiconductor chips.More specifically, the invention relates to a lithographic method.

RELATED ART

Integrated circuit technology improvements are mostly driven by thedecrease of the feature size of the semiconductor chips. As the featuresize of the circuit decreases, circuit designers have to deal with thelimitations of the lithography process used to manufacture theintegrated circuits. The lithography process starts first by coating thesurface of the semiconductor wafer with a material called resist. Asource of radiation is then shone through the mask in the case of atransparent mask. For a reflective mask the radiation is reflected bythe mask. The transparent mask is made of a substrate transparent to theradiation and coated with a patterned opaque layer defining clear andopaque regions to the radiation. Transparent masks are mostly used inoptical lithography with typical wavelengths of 436 nm, 405 nm, 365 nm,248 nm, 193 nm, and 157 nm. The reflective masks are made using asubstrate reflective to the radiation and coated with a patternednon-reflective layer defining reflective and non-reflective regions tothe radiation. Alternatively, a reflective mask could be made of anon-reflective substrate coated with a reflective layer. Reflectivemasks are mostly used for shorter radiation wavelength on the order of13 nm usually referred to as EUV or Extreme Ultra Violet.

During the exposure to the radiation source, an image of the mask isformed using an optical system on top of the resist layer. Variousoptical systems can be used to produce an image of the mask. In thetechniques called contact printing and proximity printing, the mask isplaced in contact or in close proximity to the resist. Light is shonethrough the backside of the mask thereby exposing the entireresist-coated wafer through the openings of the mask. Since the maskimage and the wafer image are of the same dimension, this technique waspartially abandoned for volume manufacturing because of the tightrequirements on the mask image and the difficulty to obtain good contactover the entire wafer area.

The main technique used today in volume production relies on theprojection of the image of the mask onto the wafer. Typically the waferimage is reduced by a factor of 4 (usually named mask imagemagnification factor or wafer image demagnification factor) as comparedto the mask image, thus relaxing the mask fabrication requirements. Thefield on the wafer corresponding to the image of the mask is exposedmultiple times to cover the entire wafer. The entire field can beexposed in one shot, in this case the equipment is named a stepper.

Alternatively, the field can be scanned by moving the mask and the waferrelative to the projection lens. In this case the equipment is named ascanner. Scanners offer the advantage to mitigate some fieldnon-uniformities observed in steppers but the scanning mechanism addsresidual noise that partially degrades the aerial image. Moreoverscanners show differences of the aerial image for features perpendicularto the scan direction versus features parallel to the scan direction. Asthe quality of the projection lenses improves, the advantage of scannersover steppers becomes less apparent.

The resist layer is exposed by the radiation passing through the mask incase of transparent mask or reflected by the mask in the case of areflective mask. The resist is then developed in a developer bath anddepending on the polarity of the resist (positive or negative), theexposed regions or the unexposed regions of the resist are removed. Theend result is a semiconductor wafer with a resist layer having a desiredpattern. This resist pattern can then be used by subsequent processingsteps of the underlying regions of the wafer.

As the feature size decreases, distortion in the pattern transferprocess becomes more severe. The design shapes must be modified in orderto print the desired images on the wafer. The modifications account forthe limitation in the lithography process. One such modification isreferred to as Optical Proximity Correction (OPC) in the case of opticallithography. In the case of OPC, modifications of the design imageaccount for optical limitations as well as mask fabrication limitationsand resist limitations. Modifications of the design image can alsoaccount for the subsequent process steps like dry etching orimplantation. It can also account for flare in the optical system aswell as pattern density variations. Another application of proximityeffect correction is the compensation of the effects of aberrations ofthe optical system used to print the image of the mask onto the wafers.In this case, a mask with aberration correction would be dedicated to agiven lithography tool as the aberrations are tool-specific.

FIG. 1 illustrates the modification of the mask data to correct forproximity effects. The processing of the mask data starts with a targetlayout 101 representing the desired dimensions of the image on thewafer. The printed image 102 of the target layout 101 differs from thedesired image due to proximity effect. For reference, the target image101 is shown with the printed image 102. The edges of the features arethen moved (103) so that the corresponding printed image on the wafer104 is correct (as close to the target as possible). In FIG. 1, all theareas of the layout have been corrected but different degrees ofproximity effect correction aggressiveness can be applied to differentregions depending on the criticality of the region in the integratedcircuit.

The corrections to layout 101 can be applied using a rule-based approachor a model-based approach. For a rule-based approach (Rule-based OPC),the displacement of the segments would be set by a list of rulesdepending, for example, on the feature size and its environment. For amodel-based approach (Model-based OPC), the printed image on the waferwould be simulated using a model of the pattern transfer process. Thecorrection would be set such that the simulated image matches thedesired wafer image. A combination of rule-based OPC and model-based OPCsometimes referred to as hybrid OPC can also be used.

In the case of model-based OPC, the original layout 201 as shown in FIG.2 is dissected in smaller segments 203 shown in modified layout 202.Each segment is associated an evaluation point 204. The printed errorsof the evaluation points are compensated by moving the correspondingsegment in a direction perpendicular to the segment as shown in thefinal layout 205. The segments are corrected using multiple iterationsin order to account for corrections of neighboring segments.

The image quality can be improved by adding printing or non-printingassist features along the edges of the main features. These assistfeatures modify the diffraction spectrum of the pattern in a way thatimproves the printing of the main feature. The practical implementationof assist features is enhanced with the use of proximity effectcorrection as described above to correct for any optical printingartifact as well as resist and etch artifacts.

The image quality can also be improved by using phase-shifting masks. Inthis case, at least two different regions are created on the maskscorresponding to different phase and transmission of the light eithergoing through these regions (for transparent mask) or reflected by theseregions (for reflective mask). The phase difference between the tworegions is chosen to be substantially equal to 180 degrees. Thedestructive interference between adjacent regions of opposite phasecreates a very sharp contrast at the boundary between the regions, thusleading to the printing of small features on the wafer. Two main classesof phase-shifting masks are in use today. For the first class, theamount of light transmitted for transparent masks (or reflected forreflective masks) by one region is only a portion of the lighttransmitted (or reflected) by the other region, typically 5% to 15%.These masks are referred to as attenuated phase-shifting masks orhalf-tone phase-shifting masks. For the second class, the lighttransmitted (for transparent masks) or reflected (for reflective masks)by one region is substantially equal to the light transmitted (fortransparent masks) or reflected (for reflective masks) by the otherregion. The second class of masks includes the following types ofphase-shifting masks: alternating aperture phase-shifting masks,chromeless phase-shifting masks, and rim phase-shifting masks. Thepractical implementation of these techniques is improved with the use ofproximity effect correction as described above to correct for anyoptical printing artifact as well as resist and etch artifacts. All thetechniques can be combined with the use of assist features.

The image quality can also be improved by using off-axis illumination.To achieve off-axis illumination, the illuminator of the stepper orscanner is shaped in a way that only the light at certain angles withrespect to the optical axis is used to create the image thereby favoringcertain spatial frequencies of the mask pattern. The off-axis settingcan be adjusted for a given feature size and type or for a collection offeature sizes and types. Off-axis illumination can be used incombination with binary masks, attenuated phase-shifting masks,chromeless phase-shifting masks, or rim phase-shifting masks. Off-axisillumination will also be improved by the use of proximity effectcorrection as described in a previous paragraph. Off-axis illuminationcan also be combined with the use of assist-features.

For the various techniques described above, i.e. proximity effectcorrection, phase-shifting masks, off-axis illumination, a very accuratemodel of the overall patterning process is required. This model stronglydepends on the optical set up of the exposure tool used to expose thewafers. In particular, the wavelength of the light source, the numericalaperture of the projection lens and the illumination setting (partialcoherence, off-axis illumination) are critical. The limitation ofcurrent optical systems is driven by the following equation:R=k ₁ λ/NA

-   -   R=resolution    -   λ=wavelength of the illumination source    -   NA=numerical aperture of the exposure system

The maximum resolution (or smallest line in a pattern made of equallines and spaces) achieved for standard optical system is achieved fork₁=0.25. But for values of k₁ below 0.5, severe distortion of thepattern can be observed on the wafer thus requiring the correction ofthe mask in order to print the desired image on the wafer.

In spite of all the techniques described above, the theoretical limit ofk₁=0.25 cannot be exceeded for current exposure tools. One approach thatwas recently proposed to extend the life of optical lithography is tocreate the image in a liquid having a high refractive index mediumthereby reducing the wavelength of the light by the reflective index.The limitation of optical systems with immersion will be driven by thefollowing equation:R=k ₁λ/(nNA)

-   -   R=resolution    -   λ=wavelength in vacuum of the illumination source    -   NA=numerical aperture of the exposure system    -   n=refractive index of the immersion medium        In this case, the k₁=0.25 limitation still holds but much        smaller feature sizes can be printed. For example, for 193 nm        wavelength, water was proposed as the immersion liquid. Water        has a refractive of 1.47 at 193 nm, thereby lowering the        smallest printable feature by a factor of 1.47. For 157 nm        wavelength, a polymer referred to as PFPE (Perfluoropolyether)        was proposed with a refractive index of 1.38. The main challenge        of immersion lithography is the manufacturing of steppers and        scanners capable of handling a film of liquid between the lens        and the wafer without creating bubble or non-uniformities. The        addition of an immersion film could result in vibration coupling        between the stage and the lens. Moreover the resist used must be        compatible with the immersion liquid. Another challenge is the        limited availability of transparent liquids with a high        refractive index at 193 nm or 157 nm.

Another option would be to use solid immersion lithography as describedin U.S. Pat. No. 5,121,256. In this case a solid immersion lens is addedto an existing optical system and placed closely adjacent to the sample.The optical system images the mask onto the wafer and the immersion lensis shaped with a spherical surface and placed at a distance such thatthe beams enter the solid lens with no refraction. For today's exposuresystems the distance between the lens and the wafer is on the order of afew millimeters while the field size image is on the order of 20 to 40millimeters thus rendering the set up described in U.S. Pat. No.5,121,256 not viable. Indeed, to image such a field size, the solidimmersion lens would be too thick compared to the distance between theprojection lens and the wafer. Moreover the set up proposed is limitedby aberrations when a large field is imaged. Building two separatelenses, one objective lens and one solid immersion lens also putsextreme requirements on the alignment of the lenses, the vibration ofthe lenses, and the overall correction of the aberrations across thefield.

Projection lenses used today for optical lithography are made of a largenumber of lens elements, typically on the order of 30 lens elements, asdescribed in U.S. Pat. No. 6,522,484. The large number of lens elementsis required in order to lower the aberration level across the field ofthe image. The shape of each lens elements is optimized by taking intoaccount all the other lens elements. Each element is accuratelypositioned inside the lens assembly and kept in an environment where thepressure, temperature and atmosphere are controlled. These tightrequirements make the use of two separate lenses as described in U.S.Pat. No. 5,121,256 impractical.

SUMMARY

The present invention provides a lithography system based on aprojection lens in which a final lens element has a surface adapted tobe placed in contact or in close proximity with the sample beingexposed, and a stage that supports the sample in contact or in closeproximity with the surface of the final lens element. The projectionlens typically comprises a plurality of lens elements including a firstlens element adapted to face a mask, and the final lens element. Thefinal lens element comprises a solid material having a high index ofrefraction, or an index of refraction greater than one. For example, insystems projecting an image using radiation having a wavelength of about193 nm, or having a wavelength of about 157 nm, the final lens elementmay comprise one of silicon dioxide, calcium fluoride, aluminum oxide,yttrium fluoride, lanthanum fluoride, strontium fluoride.

In embodiments of the invention, the plurality of lens elements of theprojection lens demagnifies an object on the mask by a factor greaterthan 4 at an image plane on or near the sample. In some embodiments, theimage plane is on or near the surface of the final lens element. In suchcases, evanescent waves from the image plane can be used to transfer theimage into the radiation sensitive layer on the sample. In yet otherembodiments, the index of refraction of the final lens element matchesthe index of refraction of the radiation sensitive layer on the sample.

The projection lens in typical embodiments comprises a plurality oflenses which are encased, to provide controlled atmosphere andtemperature in the spaces among the lenses. The final lens element has ahigh index of refraction, and is adapted to be placed in contact withthe radiation sensitive layer on the sample. In some embodiments, thefinal lens element is a slab of high index of refraction material, whichis adapted to be removed from the projection lens assembly for ease ofreplacement, and cleaning.

Further embodiments of the invention comprise a lithography system thatincludes a projection lens which has one side adapted to be placed incontact or in close proximity with the sample, and an other side adaptedto be placed in contact or in close proximity with the mask.

The present invention also provides a method for manufacturingintegrated circuits. The method includes providing a wafer having alayer adapted to be developed in response to radiation. Also, a layoutobject to be projected on the layer is provided. According to themethod, the layer on wafer which is sensitive to radiation is placed incontact or close proximity with a high index of refraction lens elementon the projection lens. The object is imaged on the radiation sensitivelayer by the projection lens.

According to embodiments of the method, the step of imaging the objectincludes imaging the object at an image plane, so that evanescent wavesemanating from the lens element transfer the image to the layer on thesample, at least in part. In other embodiments, the image of the objectis formed at an image plane near a top surface of the radiationsensitive layer. Alternatively, the image plane may be placed anywherewithin the radiation sensitive layer, according to the pattern beingtransferred, the characteristics of the material of the layer, and otherfactors.

In yet other embodiments of the invention, the method includespreventing adhesion of the lens element to the layer.

Also, embodiments the invention include placing a mask including theobject to be imaged, in contact or close proximity with another highindex of refraction lens element of the projection lens.

Embodiment of the invention includes laying out the layout pattern onthe mask to be imaged on the radiation-sensitive layer. Laying outincludes applying proximity correction using a lithography modelcomprising, for an incident material different than air characterized byits refractive index and absorption coefficient, calculating fields inthe resist, accounting for the incident material refractive index andabsorption coefficient, performed using thin film optics or by solvingMaxwell equations.

Embodiment of the invention includes laying out the layout pattern onthe mask to be imaged on the radiation-sensitive layer. Laying outincludes applying proximity correction using a lithography modelcomprising, for an incident material different than air characterized byits refractive index and absorption coefficient, calculating fields inthe resist, accounting for the incident material refractive index andabsorption coefficient, performed using thin film optics or by solvingMaxwell equations, and accounting for a gap between the incidentmaterial and the resist using thin film modeling or by solving Maxwellequations.

Embodiment of the invention includes laying out the layout pattern onthe mask to be imaged on the radiation-sensitive layer. The layoutpattern comprises an alternating aperture phase-shifting mask layout.Laying out includes applying proximity correction using a lithographymodel comprising, for an incident material different than aircharacterized by its refractive index and absorption coefficient,calculating fields in the resist, accounting for the incident materialrefractive index and absorption coefficient, performed using thin filmoptics or by solving Maxwell equations.

Embodiment of the invention includes applying an off-axis setting forthe projection lens, the off-axis setting obtained using a lithographymodel comprising, for an incident material different than aircharacterized by its refractive index and absorption coefficient,calculating fields in the resist, accounting for the incident materialrefractive index and absorption coefficient, performed using thin filmoptics or by solving Maxwell equations.

Embodiment of the invention includes laying out the layout pattern onthe mask to be imaged on the radiation-sensitive layer. The layoutpattern comprises an assist feature having a size and a distance from acorresponding main feature, and laying out includes determining saidsize and distance using a lithography model comprising, for an incidentmaterial different than air characterized by its refractive index andabsorption coefficient, calculating fields in the resist, accounting forthe incident material refractive index and absorption coefficient,performed using thin film optics or by solving Maxwell equations.

Embodiment of the invention includes laying out the layout pattern onthe mask to be imaged on the radiation-sensitive layer. The layoutpattern comprises an attenuated phase-shifting mask having sizingparameters, and laying out includes determining said sizing parametersusing a lithography model comprising, for an incident material differentthan air characterized by its refractive index and absorptioncoefficient, calculating fields in the resist, accounting for theincident material refractive index and absorption coefficient, performedusing thin film optics or by solving Maxwell equations.

Embodiment of the invention includes laying out the layout pattern onthe mask to be imaged on the radiation-sensitive layer. Laying outincludes applying proximity correction using a lithography modelcomprising, for an incident material different than air characterized byits refractive index and absorption coefficient, dividing the refractiveindices and absorption coefficients of all the materials in the waferstack by the refractive index of the incident material.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 illustrates the modification of the data to correct proximityeffects.

FIG. 2 illustrates the process flow used for model-based OPC.

FIG. 3 a illustrates an optical lithography exposure tool with theprojection lens in contact or close proximity to the wafer.

FIG. 3 b illustrates a possible projection lens used in the opticallithography exposure tool described in FIG. 3 a.

FIG. 4 depicts the interface between the lens and the resist when theyare in contact.

FIG. 5 depicts the interface between the lens and the resist in contactmode when the refractive index of the lens is matching the refractiveindex of the resist.

FIG. 6 depicts the interface between the lens and the resist in closeproximity mode.

FIG. 7 illustrates an optical exposure tool comprising a projection lensand a negative refractive index slab.

FIG. 8 depicts the interface between the lens and the resist for theoptical system described in FIG. 7.

FIG. 9 illustrates an optical exposure tool with the projection lens incontact or close proximity to the wafer and to the mask.

FIG. 10 a illustrates the methodology used to expose wafers with aglobal wafer to mask alignment.

FIG. 10 b illustrates the methodology used to expose wafers with a localwafer field to mask alignment.

FIG. 11 is a block diagram of a computer system adapted for proximitycorrection according to the present invention.

FIG. 12 is a flow chart for a process of integrated circuitmanufacturing according to the present invention.

FIG. 13 provides a system view for contact or proximity lithographyaccording to the present invention.

DETAILED DESCRIPTION

A technique described in FIG. 3 a was developed to address the issuesencountered with immersion lithography. A wafer 302 is placed on thestage 301. The projection lens 303 projects an image of the mask 304 ontop of the wafer 302. The bottom surface of the lens is placed either inclose proximity or in contact with the radiation sensitive layer(photoresist) on the wafer 302. Close proximity means that the distancebetween the bottom of the lens and the wafer is small compared to thewavelength of the exposure, typically smaller than the wavelengthdivided by 5. The set up described in FIG. 3 a will avoid the issue ofhaving a liquid between the lens and the wafer. Moreover, there is noneed to insert a solid “immersion” lens between the projection lens andthe wafer as the image of the mask 304 is created in the vicinity of thebottom surface (surface facing the wafer) of the last lens element ofthe projection lens 303. This set up allows the reduction of potentialaberrations of the images created by the lens.

A possible lens implementation is shown in FIG. 3 b. The object plane,i.e. the mask is represented by O and the image plane, i.e. the waferposition is represented by IM. The image of the object is formed at theimage plane in the vicinity of the surface of the last lens element 335(the surface in proximity or in contact with the wafer). The lenselements are placed in an environment control casing not shown on thedrawing. The temperature, pressure, and atmosphere are preciselycontrolled in the environment of the casing.

The material of the last lens element needs to be chosen with thehighest possible refractive index in order to achieve the bestresolution. At the same time the material must be compatible with allother lens requirements including for example the fact that the materialmust be transparent. Potential candidates are listed below with theirapproximate values of the refractive indices in parenthesis and comparedto liquid immersion lithography.

Wavelength Liquid Immersion Lens Materials 193 nm Water (1.47) Al₂O₃(1.88) CaF₂ (1.5) SiO₂ (1.56) YF₃ (1.55) SrF₂ (1.55) LaF₃ (1.63) 157 nmPFPE (1.38) CaF₂ (1.56) SiO₂ (1.69) YF₃ (1.64) SrF₂ (1.62) LaF₃ (1.72)

For both 193 nm and 157 nm wavelengths, the refractive indices for thelens materials are higher than the best liquid immersion materialsavailable thus enabling this technique to resolve smaller resist featuresizes than liquid immersion lithography. Some of the materials listed aspotential candidates for the lens material, i.e. CaF₂ and SiO₂, arealready used today in the manufacturing of lenses for 193 nm and 157 nmlithography. The lens manufacturing process could use the same materialas for conventional lenses, only the overall design of the lens needs tobe changed so that the image of the mask is created in the vicinity ofthe boundary of the last lens element. The exact location of the imageof the mask will need to be chosen carefully such that the image iscreated at the best location with respect to the resist. Typically theimage should be formed inside the resist. The exact location will dependon the resist thickness, on the resist refractive index, and on theresist processing characteristics. For some resist the image should beformed closer to the top of the resist, for other resists the imageshould be closer to the bottom of the resist. The difference in bestimage position can be attributed to differences in chemistry and todifferent processing conditions like for example, pre-bake time andtemperature, post-exposure bake time and temperature, developernormality, development time and temperature.

Alternatively, as current projection lenses are made of multiple lenselements, only the last element could be made of a high refractive indexmaterial in order to achieve the best possible resolution. For example alens for 193 nm lithography could use CaF₂ and SiO₂ for all the lenselements except for the last element in contact or proximity with theresist that could be made of Al₂O₃. For a 157 nm lens, all elementscould be made of CaF₂ except for last element close to the resist thatcould be made of LaF₃.

FIG. 4 shows a detailed view of the lower surface of the projection lens403 in contact with the resist 402 coated on the wafer 401. Marker 404indicates light beams propagating through the projection lens 403 andcreating an image inside the resist 402. Preferably the image of themask is created inside the resist to obtain the best resist imagequality after resist processing. FIG. 4 assumes that the refractiveindex of the lens material is lower than the refractive index of theresist and the beams at the lens/resist interface are refractedfollowing Snell's law:n ₁·sin(i ₁)=n ₂·sin(i ₂)

-   -   where n₁ is the refractive index of the lens material, n₂ the        refractive index of the resist, i₁ the angle between the beam        inside the lens material and the optical axis, i₂ the angle        between the beam inside the resist and the optical axis. The        refraction of the beams entering the resist will create an        aberration of the image inside the resist equivalent to        spherical aberration.

The aberration can be reduced if n₁ and n₂ are substantially the same asshown in FIG. 5. In this case the previous formula shows that i₁ and i₂are substantially equal. The beams propagating through the lens enterthe resist with minimal direction change and the aberration effect istherefore reduced. The lens material could be chosen to achieve a goodrefractive index match with the resist. Vice-versa, the resistcomposition could be modified to match the refractive index of the lensmaterial. The composition of the resist could also be modified so thatthe refractive index of the resist is larger than the refractive indexof the last lens element in order to ensure that no resolution lossoccurs at the interface between the resist and the lens.

If the projection lens is not in perfect contact with the resist and agap is left in between the lens and the resist, the propagation of thebeams with high angles of incidence (i₁) through the gap could bealtered if the refractive index of the gap n_(g) is lower than therefractive index of the lens material as shown in FIG. 6. Totalreflection would take place for sin(i₁)>n_(g)/n₁. The beam 605 goingthrough the lens 604 is reflected into the beam 606 because of the gap603 before it reaches the resist 602 coated on the wafer 601. In thiscase evanescent waves present at the lens/gap interface could be used toimage the resist. As their amplitude decays exponentially, the gapthickness will have to be kept small compared to the wavelength of thelight.

To alleviate the effects of exponentially decaying evanescent fields, anegative refractive index slab can be placed in between the lens and thewafer as shown in FIG. 7. A wafer 702 is placed on the stage 701. Theprojection lens 703 combined with a negative refractive index slab 705creates an image of the mask 704 on top of the wafer 702. Thepropagating fields are imaged by the slab which acts like a lens. Theevanescent fields are amplified throughout the slab and can be used onthe resist side to create the final image inside the resist.

FIG. 8 shows a detailed view of the lower portion of the projection lens806, the negative refractive index slab 804, the resist 802 and thewafer 801. A gap 805 is left between the lens 806 and the negativerefractive index slab 804 and a gap 803 is left between the negativerefractive index slab 804 and the resist 802. The dotted line 807 showsfields propagating from the lens 806, through the gaps 805 and 803 andthrough the negative refractive index material 804 to create an imageinto the resist 802. For beams with large incidence angle, gap 805results in total reflection inside the lens 806 as described in FIG. 6.Evanescent fields inside gap 805 decay exponentially but are amplifiedthrough the negative refractive index slab 804. These evanescent fieldsdecay again exponentially in the gap 803. They can create an image inthe resist if the widths of the gaps 805 and 803 are small compared towavelength of the light. Preferably the width of the gaps 805 and 803 ischosen equal to half the width of the negative refractive index slab 804and the refractive index of the slab 804 is chosen equal to −1. Thenegative index slabs can be obtained by manufacturing three dimensionalphotonic crystals.

For all the cases described above the mask is placed in air or in apurged environment for 157 nm lithography as air attenuates the 157 nmradiation. Under these conditions, the diffraction pattern emanatingfrom the mask and captured by the projection lens could be limiting theresolution of the image created on the wafer. A magnification of themask image is required if the mask is imaged in air. The conditionimposed on the magnification of the mask is as follows:m>n _(w) /n _(m)

-   -   where m is the magnification of the image on the mask compared        to the image on the wafer, n_(m) is the refractive index of the        material in contact or in close proximity with the mask and        n_(w) is the refractive index of the material in contact or in        close proximity with the wafer. In general, the resolution of        the image created on the wafer is limited by the minimum of the        refractive index of the last lens element, the refractive index        of the resist, and the refractive index of any material that        could be present in-between the last lens element and the        resist. These limitations apply to any type of optical system        currently in use and even apply to future imaging systems using        liquid immersion lithography.

Using this technique, very small features will be printed on the waferusing a relatively large wavelength as measured in the air. Theoriginating mask features will be small as well and given by thefollowing equation:MFS=mk ₁λ/(nNA)

-   -   where MFS is the mask feature size. For example, if NA=0.85,        k₁=0.3, n=1.5, m=4, then MFS=0.67λ. The mask feature size is        actually smaller than the wavelength. A magnification of 4 is        the standard for today's scanners and steppers. While the mask        feature size is small compared to the wavelength, the thickness        of the mask absorber is relatively large compared to the        wavelength, typically the chrome absorber thickness is on the        order of 70 nm for advanced masks. The high aspect ratio of the        mask features creates large scattering at the edges of the mask        feature that will degrade the mask transmission and ultimately        the quality of the aerial image printed on the wafer. Moreover        the problem becomes more acute for alternating phase-shifting        masks as the quartz substrate is etched to create a 180 degree        phase-shift by a depth given by the following equation:        d=λ/2(n _(s)−1)    -   where n_(s) is the refractive index of the mask substrate. If        the alternating aperture mask is created using an additive        process, a shifter layer is coated and patterned on top of the        chrome features. The thickness of the shifter layer is given by        the following equation:        d=λ/2(n _(sh)−1)    -   where n_(sh) is the refractive index of the shifter layer. In        the case of alternating aperture phase-shifting mask the aspect        ratio is larger and the effect of scattering is more severe. For        resolving smaller feature size compared to the exposure        wavelength, a larger magnification should be used for example 5,        6 or 7. Integer number for the mask magnification is not        required but it is preferred as it simplifies the scaling of the        wafer data to create the mask. In particular, it makes it easier        to put the mask data on a grid compatible with the mask write        tools. The limitation brought up by mask scattering effects at        the edge of high aspect ratio features and the idea of        increasing the mask magnification accordingly applies to any        types of optical system currently in use and even applies to        future imaging systems using liquid immersion lithography.

Another way to solve this problem would be to immerse the mask side ofthe imaging systems in high index liquid, or place the mask surface incontact or close proximity with a high index solid lens element, as wellas the target wafer. This can be accomplished for the optical systemsdescribed in the present invention but also for any type of opticalsystem currently in use and including future imaging systems usingliquid immersion lithography. FIG. 9 shows a drawing representing such asystem as an extension of the system described in FIG. 3 a. A wafer 902is placed on the stage 901. The projection lens 903 projects an image ofthe mask 904 on top of the wafer 902. The top of the lens is placedeither in close proximity or in contact with the mask 904. The bottom ofthe lens is placed either in close proximity or in contact with thewafer 902. This type of set up would limit the constraints on the maskmagnification described above. This type of set-up precludes the use ofa pellicle on the mask. This type of set up can also be used incombination with negative refractive index slab as described in FIG. 7.

In all the cases described in FIGS. 3 a, 4, 5, 6, 7, 8, and 9 thedistance between the lens and the wafer is set by some mechanicalconstraints. For example in contact mode as described in FIG. 4, no gapis left between the lens and the resist. The contact mode can beprecisely adjusted by monitoring the pressure exerted by the wafer stageon the lens. For FIG. 6, the gap left between the lens and the resist iscontrolled very accurately since the evanescent waves are decayingexponentially. Any non-uniformity of the gap or any small change of thegap would lead to large differences in the image formed into the resist.One method to monitor the gap distance is to use an interferometer,where the waves reflected by the surface of the resist and the wavesreflected by the surface of the lens (lens air interface) areinterfered.

As the distance between the lens and the resist is fixed because of thenature of the imaging technique used, the position of the image can bechanged by adjusting the position of the mask with respect to the lens.On the other hand as the focus position inside the resist is set by theoptical system when the wafer is in contact with the lens, a bettercontrol of the image position within the resist should be obtainedcompared to conventional exposure tool. For conventional exposure toolslike steppers or scanners, the image position within the resist, i.e.the focus setting, is adjusted by moving up or down the wafer stage withrespect to the lens which may lead to positioning inaccuracies. Inaddition the effect of vibrations or vibration coupling (liquidimmersion) should be minimized in contact mode as described in thepresent invention.

One of the main differences between this technique and currentlithography techniques is that the lens is in close proximity or even incontact with the wafer which increases the risk of contamination of thelens and of the wafer. In close proximity the risk of contamination islower but the control of the distance between the wafer and the lens iscritical. On the other hand, contact printing offers the ultimateresolution but the largest risk of contamination. In order to preventthe contamination of the lens or of the wafer, a thin layer preventingthe adhesion of the resist to the lens can be coated either on top ofthe lens or on top of the resist. This layer can be made for example ofa fluoro-polymer similar to the one used for mask pellicle. This type ofmaterial presents the advantage of being transparent to shortwavelengths and to reduce surface tension. The refractive index of thisfilm should be high enough not to limit the resolution of the printedimages by total reflection as described in FIG. 6. The resist chemistrycan also be changed to lower surface tension. Resist can be made offluoro-polymers combined with photo-acid generators. Upon irradiation,the photo-acid generator generates an acid that can de-protect chemicalfunctions on the fluoro-polymer that are soluble in the resistdeveloper. The resist would then be a positive resist. Alternativelysoluble functions could be removed (negative resist) or photo-basegenerators could be used.

In order to improve the contact between the lens and the wafer or tobetter control the gap between the lens and the wafer, the wafer surfaceshould be as flat as possible. Techniques such as chemical mechanicalpolishing can be used to improve the wafer surface flatness. Dummy fillpatterns placed in sparse areas of the layout can be combined withchemical mechanical polishing to further improve the flatness.

The contact between the lens and the wafer can also be improved by usingresists specially formulated to create a planar coating of thetopography of the wafer. The resist can also be formulated to conform tothe surface of the lens when the lens and the wafer are put in contact.

To avoid any printing degradation due to the lens cleanliness, thesurface of the lens that was in contact with the wafer can be inspectedand cleaned after a certain number of exposures. Preferably, both theinspection and the cleaning systems should be placed either on the waferstage or in the near proximity of the stage to minimize throughput loss.Many different inspection techniques can be used. A laser system can beused to scan the surface of the lens at a grazing incidence. Any changein the amount of light reflected by the surface indicates the presenceof a defect. A pinhole combined with a detector could be used to monitorthe uniformity of the aerial image of the lens. A phase measurementinterferometer could be used to monitor the uniformity of thewave-fronts across the field.

If defects have been found during the inspection step, the lens surfaceshould be cleaned. The cleaning process can combine wet and dry cleaningmethods similar to the techniques used for cleaning wafers or photo-maskin order to remove resist residues or particles. The portion of the lensthat was in contact with the wafer could be immersed in liquid chemicalcapable of dissolving the resist residues, and then dried. The wetcleaning step could be performed in combination with acoustic waves(typically 750 to 1000 kHz) in order to dislodge any particle left onthe lens surface. The portion of the lens that was in contact with thewafer could be placed in a plasma environment. The plasma could containoxygen or nitrogen as the byproducts of the reaction of polymer withoxygen or nitrogen are volatile compounds like for example H₂O, CO, CO₂in the case of oxygen. Another technique involves the use of UVradiation combined with an atmosphere of ozone. The UV radiation couldbe generated by an external source or could be generated by the exposuretool source itself.

To address the issue where the lens surface cannot be cleaned, the lenscould be built in such as way that the last element of the lens can beremoved and replaced. For example the last lens element in contact withthe wafer could be a disposable sheet of the high refractive indexmaterial.

As the image on the mask is magnified compared to the expected waferimage, multiple exposures will be required to expose an entire wafer.FIG. 10 a describes the required flow to expose and process an entirewafer. The wafer is aligned globally to the mask through an alignmentstep. The resist is first coated on the wafer, and then the wafer isloaded onto the stage. The wafer is aligned to the mask using globalalignment keys on the wafer. The stage is moved to the location of thefirst field to be exposed. The stage is then moved up in contact or inproximity with the wafer. The resist is then exposed to the desiredexposure dose. The stage is then moved down to release the wafer fromthe lens. If more fields need to be exposed the stage moves to thelocation of the next field to expose and the same procedure is repeated.When all the fields are exposed, the wafer is unloaded from the stageand the resist is processed. In FIG. 10 b, the alignment is performedfor each field individually. The flow is similar to the flow describedon the right side except that the alignment is performed just before theexposure of each field with alignment keys placed on each field.

If the lens is placed in contact with the wafer, the exposure for agiven field is performed as a whole in which the entire field is imagedby the imaging system onto the wafer (stepper mode). If the wafer isplaced in proximity of the lens, the entire field can be imaged as awhole (stepper mode) or the field can be scanned (scanner mode). Thelens elements are typically circular. To avoid neighboring fieldsoverlapping, the final lens element can be shaped into the geometry ofthe field, for example the final lens element may be rectangular. Thetypical field sizes for current stepper and scanner is a rectangle (orsquare) on the order of 20 to 40 mm in each direction. This reducedfield size (typically less than 2000 mm²) compared with the previousnon-magnified implementations (which needs to cover the entire wafer asa whole), in combination with printing with a magnified mask image (suchthat mask critical dimension tolerance is relaxed), makes contact andproximity printing described in the present invention more viable thanthe previous non-magnified implementations. The reduced field size alsomakes the resist less prone to sticking to the lens.

As far as alignment is concerned, the global or local alignment systemscan re-use existing alignment systems where the position of alignmentkeys on the mask is compared to the position of alignment keys on thewafer using an optical beam either going through the projection lens orgoing through an optical system adjacent to the projection lens.

The exposure systems described in FIG. 3 a, FIG. 7, and FIG. 9 couldalso be used to produce an image on the wafer without the need for amask. In this case, the mask would be replaced by a radiation source. Animage of the source would be created on the wafer in the form of a smallexposed area. The image of the source could be scanned on the waferusing a deflection mechanism like an acousto-optics modulator or arotating mirror placed between the source and the projection lens. Thesource could be turned on and off to create the desired pattern on thewafer. The image of the source on the wafer could have a range ofpossible shapes in order to re-create the desired wafer image.

Despite the increased resolution provided by imaging into a highrefractive index material, the distortion in the pattern transfer willrequire proximity effect correction as the feature size decreases. Thedesign shapes must be modified in order to print the desired images onthe wafer. To achieve an accurate correction of the layout, new modelsare required to account for the imaging into a high refractive index andto account for the evanescent waves coupling when the lens and wafer arein close proximity.

Modeling can be approximated using a conventional lithography simulator.Denoting the refractive index of the last lens element material by n_s,modeling can be performed using a conventional simulator by specifying awavelength given by the wavelength in vacuum divided by n_s. Refractiveindexes and extinction coefficients (n,k) of all materials in the waferstack should also be divided by n_s. Other parameters should remainunchanged. The simple approximation described above is adequate forfirst-order analysis. A more accurate modeling is necessary when theuser is interested in secondary effects such as aberrations, focuserrors, and the impact of mask topography on imaging. In thesesituations, the simulator should model the thin-film stack using n_s asthe refractive index of the incident material. Spacing of thediffraction orders would change from 1/p to 1/(n_s*p) (where p is theperiod of the printed feature) to reflect the capturing of morediffracted harmonics. For proximity or contact printing in whichevanescent waves play a non-negligible role, the model should alsoinclude extensions of thin-film optics. Alternatively, for thecalculation of the propagation through the gap between the lens and theresist and the field inside the resist, Maxwell's equations can besolved using for example the FDTD (Finite Difference Time Domain)method. With these changes, modeling of secondary effects such asaberrations and mask topography becomes accurate.

Using this methodology, an accurate model of the image process can bebuilt for both contact and proximity modes. The model built using thismethodology can be used in many applications. It can be used toaccurately correct the original layout to compensate for proximityeffects or to verify the printing of a given layout or the correction ofa layout after the compensations have been performed. This model canalso be used to calculate the image of a given layout, determine itsdose and focus latitude, or evaluate the printability of defects. Otheruses of the model are the calculations of the optimum shifter width foralternating aperture phase-shifting mask, the optimum design shapesizing for attenuating phase-shifting mask, the optimum assist featuresize and distance from the main feature, and the optimum illuminatorsetting for off-axis illumination.

FIG. 11 illustrates a computer system that can be used for one or moreof the following tasks: correcting proximity effects on data layouts,verifying the correction, simulating the image of the layout, simulatingthe optimum shifter width for alternating aperture phase-shifting masks,simulating the optimum sizing for attenuating phase-shifting masks,simulating the optimum illuminator setting for off-axis illumination.This computer system represents a wide variety of computer systems andcomputer architectures suitable for this application. A processor 101 isconnected to receive data indicating user signals from user inputcircuitry 1102 and to provide data defining images to display 1103.Processor 1101 is also connected for accessing mask layout data 1104,which define a mask layout under construction and a layout for a layerof material to be exposed using the mask. Processor 1101 is alsoconnected for receiving instruction data from instruction inputcircuitry 1105, which can provide instructions received from connectionsto memory 1106, storage medium access device 1107, or network 1108.

FIG. 12 illustrates the manufacturing process of an IC (IntegratedCircuit). At step 1201, the layout file of the integrated circuit isfirst read using a computer system described in FIG. 11. At step 1202,the layout is corrected for proximity effect and the data is convertedto mask data format. The correction of the data is only performed ifrequired to meet the lithography specifications. The data resulting fromstep 1202 is used to create a mask at step 1203, and the mask is finallyused in the fabrication process of an IC at step 1204. At least onelithographic step used to manufacture the IC in step 1204 uses one ofthe techniques described above from either, FIG. 3 a, FIG. 7 or FIG. 9.

In summary, the present invention as described in FIG. 3 a, FIG. 7, andFIG. 9 requires substantial modifications of many aspects of thesemiconductor manufacturing process. FIG. 13 summarizes some of theramifications of this technique that were described in more details inthe previous paragraphs. The use of the invention directly impacts thefollowing areas: Semiconductor-MEMS-integrated optics manufacturing,Mask manufacturing, EDA (Electronic Design Automation), ResistManufacturing, and Lithography Equipment Manufacturing.

CONCLUSION

The data structures and code described in this description can be storedon a computer readable storage medium, which may be any device or mediumthat can store code and/or data for use by a computer system. Thisincludes, but is not limited to, magnetic and optical storage devicessuch as disk drives, magnetic tapes, CD (compact discs) and DVD (digitalvideo disks), and computer instruction signals embodied in atransmission medium. For example, the transmission medium may include acommunication network, such as the Internet.

The invention can be applied to any binary masks, rim phase-shiftingmasks, chromeless phase-shifting masks, attenuated phase-shifting masks,alternating aperture phase-shifting masks used in single or multipleexposure methodologies.

While the present invention is disclosed by reference to the preferredembodiments and examples detailed above, it is to be understood thatthese examples are intended in an illustrative rather than in a limitingsense. It is contemplated that modifications and combinations willreadily occur to those skilled in the art, which modifications andcombinations will be within the spirit of the invention and the scope ofthe following claims.

1. A method for manufacturing integrated circuits, comprising: providinga sample having a layer adapted to be developed in response to radiationhaving a wavelength; providing a layout object to be projected on saidlayer; placing said layer on said sample a) in contact with a lenselement of a projection lens or b) in close proximity with the lenselement of the projection lens with a spacing between the lens elementand the sample not greater than approximately the wavelength of theradiation divided by five and without an immersion lens between the lenselement and the sample, wherein said lens element comprises a materialhaving an index of refraction for said radiation greater than 1; andimaging the object onto said layer through said projection lens.
 2. Themethod of claim 1, including imaging the object at an image plane sothat evanescent waves emanating from the lens element transfer an imageof the object to said layer.
 3. The method of claim 1, including imagingthe object at an image plane near a top surface of said layer.
 4. Themethod of claim 1, including preventing adhesion of said lens element tosaid layer.
 5. The method of claim 1, including placing a mask includingsaid layout object in contact or close proximity with another lenselement of a projection lens, wherein said another lens elementcomprises a material having an index of refraction for said radiationgreater than
 1. 6. The method of claim 1, including laying out a layoutpattern on a mask including the layout object to be imaged on saidlayer, said laying out including applying proximity correction using alithography model comprising, for an incident material different thanair characterized by its refractive index and absorption coefficient,calculating fields in said layer, accounting for the incident materialrefractive index and absorption coefficient, performed using thin filmoptics or by solving Maxwell equations.
 7. The method of claim 1,including laying out a layout pattern on a mask including the layoutobject to be imaged on said layer, said laying out including applyingproximity correction using a lithography model comprising, for anincident material different than air characterized by its refractiveindex and absorption coefficient, calculating fields in said layer,accounting for the incident material refractive index and absorptioncoefficient, performed using thin film optics or by solving Maxwellequations, and accounting for a gap between the incident material andresist using thin film modeling or by solving Maxwell equations.
 8. Themethod of claim 1, including laying out a layout pattern on a maskincluding the layout object to be imaged on said layer, the layoutpattern comprising an alternating aperture phase-shifting mask layout,said laying out including applying proximity correction using alithography model comprising, for an incident material different thanair characterized by its refractive index and absorption coefficient,calculating fields in said layer, accounting for the incident materialrefractive index and absorption coefficient, performed using thin filmoptics or by solving Maxwell equations.
 9. The method of claim 1,including laying out a layout pattern on a mask including the layoutobject to be imaged on said layer, wherein said imaging the object onsaid layer through said projection lens, includes applying an off-axissetting for the projection lens, the off-axis setting obtained using alithography model comprising, for an incident material different thanair characterized by its refractive index and absorption coefficient,calculating fields in resist, accounting for the incident materialrefractive index and absorption coefficient, performed using thin filmoptics or by solving Maxwell equations.
 10. The method of claim 1,including laying out a layout pattern on a mask including the layoutobject to be imaged on said layer, the layout pattern comprising anassist feature having a size and a distance from a corresponding mainfeature, said laying out including determining said size and saiddistance using a lithography model comprising, for an incident materialdifferent than air characterized by its refractive index and absorptioncoefficient, calculating fields in said layer, accounting for theincident material refractive index and absorption coefficient, performedusing thin film optics or by solving Maxwell equations.
 11. The methodof claim 1, including laying out a layout pattern on a mask includingthe layout object to be imaged on said layer, the layout patterncomprising an attenuated phase-shifting mask having sizing parameters,said laying out including determining said sizing parameters using alithography model comprising, for an incident material different thanair characterized by its refractive index and absorption coefficient,calculating fields in said layer, accounting for the incident materialrefractive index and absorption coefficient, performed using thin filmoptics or by solving Maxwell equations.
 12. The method of claim 1,wherein the sample comprises a wafer including a plurality of materialsforming a wafer stack, and including laying out a layout pattern on amask including the layout object to be imaged on said layer, said layingout including applying proximity correction using a lithography modelcomprising, for an incident material different than air characterized byits refractive index and absorption coefficient, dividing refractiveindices and absorption coefficients of all the materials in the waferstack by the refractive index of the incident material.